1. Field of the Invention
The present invention relates to a method for producing a transparent conductive layer comprising TiO2, which can be utilized for a Group III nitride-based compound semiconductor light-emitting element.
2. Background of the Invention
ITO (Indium Thin Oxide) has been used as a transparent electrode (transparent conductive layer) of the Group III nitride-based compound semiconductor light-emitting element to be utilized as a semiconductor light-emitting element. However, since indium contained in ITO is a rare metal, indium is a cause of increasing the cost of the transparent electrode and, as a result, the cost of the light-emitting element.
Thus, it is investigated to use Nb-doped TiO2 (“TNO” in the specification) as a transparent electrode. TNO has an advantage that a refractive index of TNO is higher than ITO and thus improvement in light extraction efficiency from the light-emitting element can be expected.
As documents intruding technologies relating to the invention, see Patent Documents 1 and 2.    Patent Document 1: JP-A-2006-66362    Patent Document 2: JP-A-2007-220970
It is well known that there are two main crystalline structures of TiO2. These are an anatase type (metastable phase) and a rutile type (stable phase), the anatase type is considered to be advantageous for obtaining a high conductivity.
According to the investigation of the present inventors, when a TiO2 layer is deposited by a sputtering method at a high temperature on a GaN surface of in a Group III nitride-based compound semiconductor light-emitting element, the TiO2 layer tends to become the rutile type, so that it is effective to use a method that an amorphous TiO2 layer deposited at low temperature is subjected to thermal treatment to cause crystallization into the anatase type. Moreover, a highly conductive film can be obtained by performing the thermal treatment in a reductive atmosphere containing hydrogen. However, when such thermal treatment is performed, an activation rate of Mg acceptors in a p-GaN layer decreases and power efficiency as a light-emitting element is influenced.
As a result of extensive studies for solving the problems, the present inventors have conceived the present invention. Namely, a first aspect of the invention is defined as follows.
A method for producing a transparent conductive layer comprising TiO2 on a Group III nitride-based compound semiconductor layer, which comprises a deposition step of depositing an amorphous TiO2 layer on the Group III nitride-based compound semiconductor layer and a thermal treatment step of thermally treating the amorphous TiO2 layer in an atmosphere where hydrogen gas is substantially absent to thereby crystallize the TiO2 layer.